Access Python attributes at construction time, C++ side

[adriendelsalle]

Description

Hi! Thanks for this great project!

I'm trying to use a trampoline class to add/modify Python attributes at construction time, C++ side.
This is a pretty handy feature to be able to add dynamically or init some attributes at construction time.

Reading the docs I was expecting this to be already the case:

Sometimes, however, it is useful to always initialize a trampoline class as an intermediate class that does more than just handle virtual method dispatching. For example, such a class might perform extra class initialization, extra destruction operations, and might define new members and methods to enable a more python-like interface to a class.

I didn't managed to make it work from master. Here is a minimal example to explain the intent:

• C++ code

// main.cpp
#include "pybind11/pybind11.h"
#include "pybind11/stl.h"

namespace py = pybind11;

class Base {
public:
  Base() = default;
  virtual ~Base() = default;
};

class BaseTrampoline : public Base {
public:
  BaseTrampoline() {
    auto pyobj = py::cast(this);
    // get/set Python attributes
    pyobj.attr("bar") = 10.;
    for (auto &n : pyobj.attr("__inputs__").cast<std::vector<std::string>>())
      pyobj.attr(n.data()) = n;
  };
};

PYBIND11_MODULE(test, m) {
  py::class_<Base, BaseTrampoline>(m, "Base").def(py::init<>());
}

• Python test script

# test.py
from test import Base

class Derived(Base):
    __inputs__ = ["a", "b"]

d = Derived()

assert d.bar == 10.0
assert d.a == "a"
assert d.b == "b"

• create a CMakeLists.txt file

# CMakeLists.txt
project(test)

find_package(pybind11 CONFIG REQUIRED)

pybind11_add_module(test main.cpp)
install(TARGETS test DESTINATION ${CMAKE_SOURCE_DIR})

• build/install the project

mkdir build
cd build/
cmake ..
make install

Running the test script python test.py gives an AttributeError:

Traceback (most recent call last):
  File "<...>/test.py", line 8, in <module>
    d = Derived()
AttributeError: 'test.Base' object has no attribute 'bar'

As far as I understand, the py::cast(this) doesn't behave correctly because the C++ object is not yet allocated and registered.
To make it possible (correct me if I'm wrong..), the following changes would be necessary:

• preallocate the memory and register the C++ value_ptr
• construct the object using a placement new operator right after

I saw that placement new op was removed earlier to simplify the syntax when using a custom new operator (see #948 and #805).

Maybe a good option is to keep the current behavior as is and pass an extra option (py::preallocate) for people willing to do the described behavior (in that case it would probably need to add placement new operator if needed)?

I started some tries, and would be happy to open a PR to discuss on changes!